Most high power diode lasers are edge emitting semiconductors, with the laser emitted from one facet. Typical dimensions of one diode laser are a facet of 500 microns width and 100 microns height with a length between 0.6 mm to 3 mm. The optical brightness of the diode laser is defined by its internal structure. Parallel to the pn-junction (fast axis) the emission is diffraction limited and emerging from an aperture of about 1 micron with a divergence of typical 30 degrees (half angle for 1/e2), in a direction parallel to the mounting surface of the diode laser. Modifications of the internal structure of the diode laser allow lower angles of divergence, e.g. by increasing the aperture (so called Large Optical Cavity (LOC) structures). In the axis perpendicular to the pn-junction, the light is emitted from an aperture in the range of several microns to 200 microns with a beam quality defined by the size of the aperture. For typical 3-5 microns large aperture the emission is also diffraction limited in the slow axis, but for larger apertures, the emission is no longer diffraction limited and the beam quality typically decreases with larger aperture. A typical divergence of a 100 micron wide aperture is 4 degrees (half angle for 1/e2).
High power diode lasers with output powers of 100 W and more are realized by arranging multiple edge emitting diode lasers, so called single chips, next to each other in one device, so called array. In such cases, special measures have to be taken to provide for an efficient dissipation of heat generated by the multiple diode lasers.
Alternate structures of high power diode lasers are semiconductor devices with integrated mirrors to deflect the emission parallel to the pn junction to emerge perpendicular to the mounting surface of the diode laser. Vertical Cavity Surface Emitting Lasers (VCSEL) represent another group of devices as well as structures with higher order Distributed Bragg Grating (DBG).
Focusing the light of edge emitting diode lasers to a small spot requires optical elements for collimation and focusing. However, it is noted that the beam quality of such a high power laser diode is highly asymmetric. Typically, in fast axis the beam quality is diffraction limited in fast axis, characterized by M2=1 and in slow axis the beam quality for a 100 micron broad aperture is in the range of M2=16. The issue of asymmetric beam quality in fast and slow axis is much more severe for high power arrays, with multiple single chips arranged in one semiconductor next to each other in slow axis direction. At a beam quality of M2=1 in fast axis, the beam quality in slow axis can decrease to M2=1,000. Because the output laser beam is highly asymmetric, typically two collimating steps are performed in the prior art. Typically first micro-optical lenses are used for collimating the highly divergent beam in fast axis and secondly, collimating lenses for slow axis collimation are deployed resulting in a beam collimated in both axes that can subsequently be focused with one or more lenses to a small spot. Nevertheless, the symmetry of said collimated output laser beams is not satisfactory for many applications. Accordingly, there exists a need to provide simple and cost-efficient solutions to enable high power laser diodes to output laser beams of high beam quality, in particular high symmetry.
To optimize the brightness, defined as the power from a given aperture emitted in a specific space angle, the beam quality must be symmetrized in fast and slow axis. Several concepts have been developed for diode laser arrays in the past. State of the art solutions use refractive or reflective optics to cut the emission in slow axis in several sections with subsequent rearranging in fast axis. Because of the high divergence angle in the fast axis direction, all these approaches dispose a collimating lens for fast axis collimation at very short distances from the emitting facet of the laser diodes, i.e. make use of collimating lenses of short focal length for fast axis collimation. This approach usually requires precise alignment of an array of multiple micro-optical lenses in six axes, which makes the whole setup relatively complex and expensive. Nevertheless, a substantial loss of beam quality is experienced because of unavoidable tolerances in the parts, such as smile of the lens or diode array, as well as in the alignment of the laser light emitter(s) to the associated optical component(s).
For single chips symmetrizing the beam quality has not been explored. The high effort for precision alignment of the single chip or the respective optics and the associated costs prohibit further exploration.
US 2003/0048819 A1 discloses a laser diode array comprising a plurality of multi-cavity laser diode chips fixed side by side. According to a first embodiment, shown in FIG. 11 to 13, the laser diode chips are soldered onto a substrate. In order to position the laser diode chips, a reference mark is provided of an upper surface or side surface of the substrate. In this embodiment, the laser diode chips are not aligned in fast axis direction. Disclosed is also a second embodiment (FIG. 14 to 16), wherein several of such laser diode arrays are stacked one upon the other. In this embodiment, the laser diode chips are aligned in fast axis direction. In this embodiment, the substrate, which is a heat sink made of copper, has a stepped profile. At the front face of each step there is provided a recess for preventing interference with the laser diode array fixed to the lower substrate. At the front face of each step a collimator lens array is fixed.
The precise distance between the laser diode chips and the lens array relies on exact positioning of the laser diode chips with regard to the front edge of the recesses. The tolerances caused by machining the stepped profile and the recesses are too high for precise fast axis collimation.
US 2004/0114648 A1 discloses laser diode array corresponding to the second embodiment of afore-mentioned US 2003/0048819 A1.
U.S. Pat. No. 5,715,264 discloses the stacking of laser diode submounts in fast axis direction. At the front facet of the laser diode chips there is provided a cylindrical microlens
U.S. Pat. No. 5,099,488 discloses a laser array sub mounts structure, wherein the laser diode chips are mounted upright and close to each other. A lens array with a period that is equal to the period of the laser diode chips is disposed in front of the front facets of the laser diode chips. Precise alignment of the lens array to the laser diode array is not disclosed.